1. Field of the Invention
The present invention relates to a duplexer that separates and extracts a transmission signal and a reception signal having different frequencies, to conduct transmission/reception using a common antenna.
2. Description of the Related Art
A device having a two-way radio communication function such as a cellular phone, and a radio communication system using this type of device as a communication terminal can conduct transmission/reception using one antenna included in the communication terminal, so that they provide a difference between a frequency of a transmission radio wave and a frequency of a reception radio wave and separate a transmission signal and a reception signal in a duplexer using the frequency difference.
A duplexer is formed of a receiving-side filter that selects a frequency of a reception radio wave and outputs the radio wave to a reception processing section in a device, and a transmitting-side filter that selects a frequency of a transmission signal from a transmission processing section in the device and outputs the signal to an antenna, and has a center frequency different from that of the receiving-side filter.
FIG. 13 shows a configuration example of a generally used duplexer 100. In the duplexer 100 in this example, a high band side filter 4 that selects a frequency of a reception radio wave (for example, center frequency fR=2,140 MHz) and outputs the radio wave to a reception processing section (illustration is omitted) in a device and a low band side filter 3 that filters (selects a frequency of) a transmission signal (for instance, center frequency fT=1,950 MHz) from a transmission processing section (illustration is omitted) in the device and applies the signal to an antenna port 2, are integrally provided with respect to one antenna port 2.
As above, the duplexer 100 is formed of the low band side filter 3 and the high band side filter 4 connected in parallel with respect to the antenna port 2, in which it exhibits a high band side filter characteristic 112 that includes a high band side frequency pass band 115 shown in FIG. 17 with respect to a received current that flows between the antenna port 2 and a high band side filter port 6 (which corresponds to an output port of the high band side filter 4) provided at an outlet of the high band side filter 4, and exhibits a low band side filter characteristic 111 that includes a low band side frequency pass band 114 with respect to a transmitted current that flows between a low band side filter port 5 (which corresponds to an input port of the low band side filter 3) at an inlet portion of the low band side filter 3 and the antenna port 2. Further, by improving an isolation characteristic (separation characteristic) 113 in which no frequency pass band exists between the low band side filter port 5 and the high band side filter port 6 in the two filters 3 and 4, the transmission/reception signal is designed not to flow into the filters 3 and 4 in the direction opposite to the designed direction.
Some of the low band side filter 3 and the high band side filter 4 composing the duplexer 100 having such a function adopt a ladder-type filter that connects small-sized and low-loss elastic wave resonators such as, for instance, SAW (Surface Acoustic Wave) resonators in a ladder shape (type). For example, the low band side filter 3 shown in FIG. 13 is formed of a six-stage ladder-type filter in which three series arms 31a, 31b and 31c each formed of the elastic wave resonator are serially coupled in this order from the low band side filter port 5 toward the antenna port 2, and parallel arms 32a, 32b and 32c each formed of the elastic wave resonator are respectively connected to a front stage of the series arm 31a, between the series arms 31a and 31b respectively positioned on a first stage and a second stage, and between the series arms 31b and 31c respectively positioned on the second stage and a third stage.
Meanwhile, the high band side filter 4 shown in FIG. 13 is also formed of a six-stage ladder-type filter in which series arms 41a, 41b and 41c each formed of the elastic wave resonator are serially coupled in this order from the antenna port 2 toward the high band side filter port 6, and parallel arms 42a to 42c formed of the elastic wave resonators are respectively coupled between the respective series arms and to a rear stage of the series arm 41c in the same manner as in the low band side filter 3. Specifically, each of the low band side filter port 5 side and the high band side filter port 6 side in the respective filters 3 and 4 is configured as a π-type circuit in which the parallel arms are connected to the series arms in the directions of the respective filter ports 5 and 6 sides, and each of the parallel arms 32a to 32c, and 42a to 42c is grounded.
FIG. 14 is an example of a chip that forms the duplexer 100, in which the low band side filter 3 and the high band side filter 4 composing the ladder-type filters are respectively formed on square-shaped piezoelectric substrates 11 and 12, and the piezoelectric substrates 11 and 12 and each of later-described piezoelectric substrates 10 are made of, for instance, LiTaO3, LiNbO3, crystalline quartz or the like. The reason why the filters 3 and 4 are formed on the separate piezoelectric substrates 11 and 12 as shown in FIG. 14 is as follows. In the duplexer, a high attenuation is required in the isolation characteristic 113 between the low band side filter port 5 and the high band side filter port 6. However, since a relative dielectric constant in the piezoelectric substrate is higher than that of air, if the high band side filter and the low band side filter are formed on the common piezoelectric substrate, an electrical coupling of each of elements between the filters may occur due to a stray capacitance inside the piezoelectric substrate. Accordingly, the high band side filter 4 and the low band side filter 3 are formed on the separate piezoelectric substrates and the influence on the isolation characteristic 113 is suppressed by reducing the influence of the stray capacitance, to thereby fulfill the requirement of the high attenuation.
If an upper side and a lower part in the drawing are respectively defined as a rearward side and a forward side, a low band side filter antenna port 2a composing the antenna port and the low band side filter port 5 are respectively formed on a rearward of the right side and a forward of the left side of the piezoelectric substrate 11, and are connected to the series arms 31a to 31c and the parallel arms 32a to 32c composing the low band side filter 3 via a connecting line 103 patterned on the piezoelectric substrate 11. The low band side filter antenna port 2a and a later-described filter antenna port 2b are electrically connected to each other via a conductive path provided in a package on which the piezoelectric substrates 11 and 12 are mounted, and form the antenna port 2. Further, 33a to 33c in the drawing denote ground ports for grounding the parallel arms 32a to 32c, and the ports are provided on the downstream sides of the respective parallel arms 32a to 32c. An input section of the transmission signal to the series arm 31a is connected to the low band side filter port 5, and through which the transmission signal from a not-shown transmission processing section is input into the low band side filter 3.
A high band side filter antenna port 2b and the high band side filter port 6 are respectively formed on the rearward side and the forward side of the piezoelectric substrate 12, and are coupled to the series arms 41a to 41c and the parallel arms 42a to 42c composing the high band side filter 4 by a connecting line 104 patterned on the piezoelectric substrate 12. Further, 43a to 43c in the drawing denote ground ports for grounding the parallel arms 42a to 42c, and the ports are provided on the downstream sides of the respective parallel arms 42a to 42c. 
The reason why the filter antenna ports 2a and 2b are respectively disposed on the rearward side of the low band side filter port 5 and the high band side filter port 6 in the piezoelectric substrates 11 and 12 is to enlarge a distance between the ports in the piezoelectric substrates for preventing a short circuit between the filter antenna port 2a and the low band side filter port 5 and between the filter antenna port 2b and the high band side filter port 6, respectively. Further, the antenna port 2 in a later-described drawing is also disposed on the rearward side of the low band side filter port 5 and the high band side filter port 6 in the piezoelectric substrate 10 by the same reason.
The respective piezoelectric substrates 11 and 12 are mounted on the package in a layout shown in FIG. 14, namely, in a state of being arranged side to side so that the respective filter antenna ports 2a and 2b come close to each other and the distance between the filter ports 5 and 6 becomes large. This is because the improvement of characteristic of each filter can be realized by decreasing the distance between the filter antenna ports 2a and 2b while preventing the short circuit between the filter ports 5 and 6.
Each of the SAW resonators 31a to 31c, 32a and 32b, 41a to 41c, and 42a and 42b is formed of a piezoelectric substrate and an electrode part 7 formed by being patterned on the piezoelectric substrate, in which each of the electrode parts 7 includes a well-known IDT electrode 71 shown in FIG. 15(a) and reflectors 72 disposed on the left and right of the IDT electrode 71. Note that in the respective drawings such as FIG. 14 which will be described hereinbelow, the IDT electrode 71 and the reflectors 72 are illustrated in a simplified form as shown in FIG. 15(b). Further, for easier identification, in the respective drawings to be described hereinafter, an identification code “s” is appropriately attached to the series arms 31a to 31c and 41a to 41c, and an identification code “p” is appropriately attached to the parallel arms 32a to 32c and 42a to 42c. 
Incidentally, in accordance with a further miniaturization of the cellular phone and the like in recent years, the duplexer of a smaller size has been also required, and thus there is a necessity to form two ladder-type filters into one chip, to thereby form them on one piezoelectric substrate, as shown in FIG. 16, for instance.
In this example, the piezoelectric substrate 10 is formed symmetrically, and in a region on the left side of the drawing, the series arms 31a to 31c, the parallel arms 32a to 32c, the respective ground ports 33a to 33c, and the low band side filter port 5 composing the low band side filter 3 are disposed in the same layout as that of the piezoelectric substrate 11. Further, in a region of the piezoelectric substrate 10 on the right side of the drawing, the series arms 41a to 41c, the parallel arms 42a to 42c, the respective ground ports 43a to 43c, and the high band side filter port 6 composing the high band side filter 4 are disposed in the same layout as that of the piezoelectric substrate 12.
Further, in each of regions on the left side and the right side of a center of the piezoelectric substrate 10 in the left and right directions thereof, conductive paths 105 are formed in substantially the same layout as that of the connecting lines 103 and 104 respectively formed on the piezoelectric substrates 11 and 12, the conductive paths 105 are joined at a center of the piezoelectric substrate 10 in the left and right directions thereof, and connected to the antenna port 2 by directing toward the rearward side of the piezoelectric substrate 10. The detailed layout of the series arms, the parallel arms and the respective ports is the same as that of a duplexer 1 according to an embodiment of the present invention, so that it will be described in the embodiment.
If the duplexer 100 is formed on one piezoelectric substrate 10 as described above, a coupling is occurred between the electrodes composing the SAW resonators in the respective low band side filter 3 and the high band side filter 4 and between the electrode of the SAW resonator in the high band side filter 4 and the electrode of the SAW resonator in the low band side filter 3 as shown by C1 to C15 in FIG. 18, for instance, due to the stray capacitance inside the piezoelectric substrate 10, and therefore, unnecessary paths are formed. As a result of this, the isolation characteristic 113 at the high band side is deteriorated, which is a problem. Note that even when the low band side filter 3 and the high band side filter 4 are formed on the respective two piezoelectric substrates 11 and 12, if the piezoelectric substrates 11 and 12 are reduced in sizes and a distance between the elements is reduced or a distance between the substrates is reduced, such coupling shown in FIG. 18 may occur, resulting that the isolation characteristic may deteriorate.
Patent Document 1 discloses a duplexer, in which a parallel arm SAW resonator that forms a low band side filter and is closest to a low band side filter port is connected to a parallel arm resonator at a high band side, so that the duplexer has a different configuration from that of the present invention. Further, in a duplexer disclosed in Patent Document 2, parallel arm SAW resonators are not connected to each other, so that the duplexer has a different configuration from that of the present invention. Besides, these Patent Documents make no reference to the aforementioned problem regarding the coupling, and thus it is not possible to solve the above-described problem in the inventions disclosed in these Patent Documents.
[Patent Document 1] Japanese Patent Application Laid-open No. 2002-368572 (FIG. 2)
[Patent Document 2] Japanese Patent Application Laid-open No. 2007-189501 (FIG. 4)